The present invention is directed to interval timers.
There are many applications in which it is desired to measure the time interval between two events. The occurrences of the events are typically indicated by start and stop trigger signals. A common method of making such a measurement is to provide a counter and a high-frequency clock signal and to gate the clock signal to the counter with a gating signal that begins on the occurrence of the start trigger and ends on the occurrence on the stop trigger. The counter output at the end of the timing interval is then an indication of the length of the interval.
In this method, it is the period of the clock signal that determines the resolution with which the interval can be measured. Speed limitations of available counters impose limitations on the shortness of the clock period. However, various methods have been employed to increase the resolution over that provided by the counter output alone. In most such arrangements, a high-current source charges a capacitor from the beginning of a clock interval until the occurrence of a trigger pulse. The capacitor is then discharged with a small current, and the discharge interval is timed. This "stretches" the interval between the last clock pulse and the trigger signal so that the interval is measured with greater resolution.
Unfortunately, pulse-stretching techniques result in significant dead time; the system cannot make a new interval measurement while the capacitor is slowly discharging. Moreover, measurements made near the beginning of a clock-pulse interval are subject to inaccuracies because switching transients can cause non-linearities in capacitor charging.